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The MBTA general manager told the agency's board of directors on Thursday that an inspection of its new Orange and Red Line subway car fleet had revealed bolts were improperly installed in the brakes of seven additional train cars.

The trains, which are assembled in Springfield, Massachusetts by Chinese-owned manufacturer CRRC, were pulled from service on May 19, after a braking issue caused one of the new Orange Line trains to become disabled.

"What we found was that a single bolt on the braking unit had not been properly installed, so we went through an exhaustive inspection process across this fleet to verify that all bolts were properly installed," General Manager Steve Poftak said.

"Because these cars are our newest cars in the fleet, we really wanted to understand what happened and make sure that there was no there was no larger issue here," Poftak said.

"We have currently inspected all of the Red Line cars and almost all of the Orange Line cars," Poftak said.

"We have found seven cars where the bolts were out of the specified torque range," Poftak said. "In many cases, it was not visually obvious that something had occurred, but just they were out of the specified torque range."

Poftak said there are 144 bolts like this on each train.

Video: Sky 5 shows dozens of parked MBTA train cars in Wellington yard

On Monday, when the MBTA announced the trains were returning to service, the transit agency had said the issue had not been found in any other braking units at that point.

"I just want to assure everyone that we have multiple braking units on every train and this was not a threat to safety," Poftak said. "This was a mechanical problem."

Poftak said the transit agency was holding the seven cars out of service for the time being for further analysis, while the remaining cars that passed inspection were placed back into service.

"We're going to send the bolts in question out for analysis and we have also put in changes to the assembly process ... to verify that all these bolts are properly installed," Poftak told the board.

"Going forward, we're also putting in an additional level of inspection of these bolts so that we can ensure that this problem does not reoccur," Poftak said.

The new trains have been pulled from service several times before.

Just over a year ago, a derailment at Wellington Station caused all of the new vehicles on the Orange Line to be pulled off the tracks for about five months.

In 2019, they were temporarily pulled out of service for the replacement of door components and again after engineers observed an unexpected noise coming from the underside of the vehicles.

Other cars operating on the Orange Line were originally put into service from 1979 to 1981.

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